Radio receiver



Aug- 11, 1936- H. A. WHEELER 2,050,679

#HAM/56AM RADIO RECEIVER Harald Weeier Mdm ATTORNEYS Aug. 1l, 1936. H. A. WHEELER RADIO RECEIVER 6 Sheets-Sheet 2 Filed OCT.. 3, 1953 a aw W N M mw E y ww E, .ww E e ...Ew E Q M a,

ATTORNEY5 Aug' 11, 1936- H. A. WHEELER 2,050,679

RADIO RECEIVER Filed Oct. 3, 1933 6 Sheets-SheefI 3 J0 Wifi INVENTOR Harold Wee/,er

ATTORNEYS Aug. 11, 1936. H. A. WHEELER RADIO RECEIVER Filed Oct. 3, 1933 6 Sheets-Sheet 4 EE v M mw M -M m @Wm ATTORNEY-5 Aug. 11, 1936.

RADIO RECEIVER Filed Oct. 3, 1953 H. A. WHEELER e sheets-sheet. 5

ATTORNEYS Aug.. 11, 1936. H. A. WHEELER RADIO RECEIVER 1953 6 Sheets-Sheet 6 Filed Oct. 3,

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l |00 49o FHEGQENCY (CYCLES) Patented Aug. 11, 1936 UNITED STATES PATENT OFFICE RADIO RECEIVER Application October 3,

44 Claims.

This invention relates to carrier wave signal-f?la ing and more particularly to the distortionlessgand selective reception of radio signals free from interfering signals or disturbances.

The invention is particularly applicable to the reception of modulated carrier signals, as commonly employed in radio broadcasting, vwherein the radio-frequency carrier Wave is modulated by a modulation, or audio, frequency so that the transmitted signals comprise the radio-frequency carrier together with two associated sidebands of modulation.

According to this invention, the desired distortionless and selective reception is provided by the use of apparatus which selects and translates in its entirety only one sideband of the modulated carrier signal. Since only one sideband, instead of the usual two, is entirely utilized, the signal se-lecting circuits of the receiver may be designed to pass only about one half the band of frequencies which would be required in the case of vthe usual double sideband reception. A smaller selected band width thus serves to eliminate unde-. sired signals and minimize interference.

Although the single sideband type of reception may be utilized with other types of receivers, it is particularly applicable to receivers of the superheterodyne type; and most of the detailed description and drawings relate specically to a superheterodyne single sideband receiver.

A further advantage of the single sideband type of reception is that either one or the other of the two sidebands normally transmitted by a transmitting station, may be selected at will. Hence, in the event interfering signals happen to be encroaching upon the frequencies of one of the sidebands, a slight adjustment of the tuning dial will select the other sideband, which in all probability will be free from interference. It is very unusual for both sidebands of a transmitting station to be subject to interference from other stations.

A feature of the present invention is that the signal selecting circuits are preferably adjusted so that the radio signal carrier frequency is located at either one edge or the other of the narrow modulation band which is permitted to be passed. This means that the selecting circuits, instead of being tuned symmetrically to the carrier frequency, as is usually the case, are tuned to the mid-frequency of the sideband being utilized.

A particular feature is a system of selective admission which increases the audible response of the receiver sharply to a maximum when the receiver is unsymmetrically tuned in the proper 1933, Serial No. 691,927

(Cl. Z50- 20) condition to uniformly receive only the carrier and one sideband. This is preferably accomplished by cooperation between a selective trap network and the automatic volume control.

Another feature resides in the form of automatic volume control system associated with the receiver. The automatic Volume control is of a suspended type, whereby the control operates only after a predetermined signal strength is reached, and its operation is independent of modulation. f

4'I'he output of the receiver is maintained unusually uniform by virtue of a reversed automatic volume control, associated with the automatic volume control system, which provides the required ultimate variation of bias' voltage for gain control.

A further feature isthe use of a quieting system by reason of which the receiver is maintained unresponsive unlessthe received signal strength vexceeds a predetermined value, and unless the receiver is correctly tuned.

Both theautomatic volume control and quieting systems referred to above are provided with a time lag which is comparable with Vthe period of the lowest audio or modulation frequency. The time lag of the quieting system is made as great, or greater than, the time lag of the automatic volumecontrol.

The above and other features of the invention will become apparent from the following detailed description in conjunction with the accompanying drawings.

Of the accompanying drawings, Figs. la-l'f illustrate graphically the selectivity characteristics of double and single sideband radio receivers;

Figs. Zal-2d similarly illustrate the selectivity characteristics of double and single sideband superheterodyne receivers;

Fig. 3 is a diagram showing in generalized form a single sideband receiving system of the superheterodyne type, in accordance with this invention;

Fig. 4 is a schematic circuit diagram of a receiver constructed in accordance with the diagram of Fig. 3;

Figs. fla-4f illustrate symbols and conventions used in the schematic diagram of Fig. 4;

Fig. 5 shows graphically the alignment errors of the local oscillator in the system of Fig. 4, relative to the radio-frequency amplifier;

Fig. 6 shows graphically howthe transconductance and the modulation of the modulator vary with change of control grid bias in the superheterodyne receiver of Fig. 4;

Fig. '7 shows graphically the selectivity of the intermediate-frequency amplifier of Fig. 4;

Fig. 8 shows graphically the variation of the receiver output with angular rotation of the manual volume control;

Fig. 9 shows graphically the fidelity of the audio amplifier of Fig. 4 and illustrates the compensation in the audio amplifier for the loss of part of a sideband in the receiver;

Fig. l0 shows the change of audio-frequency fidelity upon change of Volume level;

Fig. 1l illustrates the relative gain or responsiveness of the intermediate-frequency trap of Fig. 4, which creates the selective admission feature;

Fig. 12 shows the change of automatic volume control bias voltage with change of intermediatefrequency carrier voltage, in the receiver of Fig. 4;

Fig. 13 illustrates the automatic volume control and quieting of the audio-frequency output of the receiver of Fig. 4; and

Figs. 14 and 15 show the selective admission and related quieting of the receiver of Fig. 4.

A transmitted radio telephone signal comprises a carrier frequency which is modulated at audio frequencies. This modulation does not alter the carrier frequency component of the signal Wave, but merely superimposes thereon two symmetrical bands of frequencies which are known as sidebands. The lower of these sidebands contains, for each audio frequency of modulation, a component whose frequency is equal to the difference of the carrier and audio frequencies, and the upper sideband contains, for each audio frequency of modulation, a component whose frequency is equal to the sum of the carrier and 'audio frequencies. Ordinarily, no single frequency component of a sideband can exceed half the intensity of the carrier frequency; that is to say, the sum of two symmetrical sideband components cannot ordinarily exceed the carrier frequency intensity, otherwise the modulation would exceed 100%. Each sideband individually represents half the total modulation of the signal, and therefore contains a complete picture of all the modulations, even though at a reduced intensity as compared with the total modulation represented by both sidebands.

Since in ordinary broadcast reception, the carrier frequency and both sidebands are utilized, there is required for such reception a receiver which selects and passes a frequency band whose width is double the highest audio frequency utilized in the reception. 'I'his considerable band width places a limitation upon the selectivity of the receiver against interference from undesired signals, even in instances where one of the sidebands is entirely free from interference.

Single sideband reception is a method of reception which utilizes completely only one of the two sidebands, although the carrier frequency and the inner edge (representing the lower audio frequencies) of the other sideband may also be utilized to advantage. For any given signal, this type of reception requires a receiver which selects and passes a frequency band of width only equal to the highest audio frequency utilized in the reception. As a result, a single sideband receiver can be greatly improved in selectivity as compared with a double sideband receiver, and this improvement is not accompanied by the loss of any of the desired audio frequencies of modulation.

To furnish a better understanding of the invention, the operation of simple double sideband and singe sideband receivers will be briefly described before introducing the single sideband receiver of this invention. The general operation and relationships of such receivers are described with reference to Figs. 1 and 2, the following symbolic notations being employed:

fa=an audio frequency of modulation.

fb=the highest required audio frequency of modulation=the width of one sideband.

.fc--a carrier frequency in general.

fs=a radio signal frequency in general.

f=the nominal tuning frequency indicated on the dial of the receiver=the center frequency of the signal frequency band determined by all selectors in the receiver.

fsczthe signal carrier frequency.

fsa=fsc-fa=a lower side frequency, a component of the lower sideband.

fsa"=fs+fa=an upper side frequency, a component of the upper sideband.

fsb=s-fzz=the lower boundary of the lower sideband.

fsb"=fsc+fb=the upper boundary of the upper 2 0 sideband.

Fig. l shows a number of graphs, each plotted with frequency indicated along the horizontal axis, the ordinates in the case of Fig. la representing relative component frequency intensities, and in the case of Figs. lb to lf, inclusive, representing relative gain or responsiveness. In Fig. la, graph I0 'represents' the frequency spectrum of audio-frequency telephone currents, and graph II represents the carrier and sideband spectrum of a radio-frequency signal wave modulated by these audio-frequency currents.

Fig. 1b shows the selection characteristics of an ordinary double sideband receiver. Graph I 2 shows the responsiveness of all the selector circuits combined, the responsiveness being uniform over both sidebands and falling off abruptly beyond the extreme frequencies of each sideband. Graph I3 shows the responsiveness of the receiver to audio modulation, this being determined by the two symmetrical portions of graph I2 at either side of the signal carrier frequency fst, as effecting the respective sidebands. From these graphs, it is observed that a double sideband receiver having these selection characteristics is uniformly responsive to audio modulation within the required range of audio frequencies.

In Fig. le, graphs I4 and I5 show the selection characteristics for signal and audio frequencies, respectively, of a double sideband receiver which is made too selective. Graph I4 is only half as wide, in the frequency scale, as graph I2, which causes the outer half of each sideband to be cut off. Graph I5 shows the audio-frequency effect that the receiver is unresponsive to modulation at the higher audio frequencies.

Fig. 1d shows the selection characteristics of a single sideband receiver operating according to this invention. Graph I6 is the same as graph I4 but is displaced in the lower frequency direction by an amount approximately equal to half the width of a sideband. 'Ihis is effected by adjusting the signal selective circuits off center (with respect to the signal-carrier frequency fsa), so as to include all of the lower sideband and exclude most of the upper sideband. Graph II shows the resulting effect upon the audio frequency, namely, that the receiver is more or less responsive to modulation at all required audio frequencies. The low audio-frequency modulation is carried by side frequencies very near the carrier frequency where the receiver is responsive on both sides of the carrier, whereas the high audio-frequency modulation is carried by the outer side frequencies where the receiver is responsive on only one side of the carrier. The responsiveness to high audio frequencies is, therefore, reduced to half of what it would be in the case of double sideband reception. This deficiency of the higher audio frequencies may be readily corrected, however, by doubling the gain of the audio amplifier at the higher audio frequencies, the resulting uniform response being illustrated by graph I8.

Fig. 1e is similar to Fig. 1d except that graph I9 shows tuning off center (with respect to the signal carrier frequency) in the opposite direction from that shown in Fig. ld, so that the upper, instead of the lower, sideband is passed. The resulting audio-frequency response, shown by graphs II and I8, is the same as in the case represented by Fig. 1d.

Fig. 1f shows a method for obtaining uniform responsiveness in a single sideband receiver, without compensating correction in the audio amplifier. Comparing with Fig. 1d, graph 20 is not as wide as graph I6, although the detuning from the carrier frequency ,fsc is just as great; hence, the receiver is only half as responsive at the carrier and the immediately adjacent side frequencies as in the case of Fig. 1d. For any given audio frequency of modulation, the total responsiveness at all side frequencies is the same, as shown by graph 2|.

The operation of double sideband and single sideband reception as applied to superheterodyne receivers will be described with reference to Fig. 2. For the purpose of such description, the following additional notation is employed, which assumes that the superheterodyne oscillator frequency is greater than the received carrier signal frequency, but less than double the signal frequency.

ffzan intermediate frequency in general.

ff0=the nominal intermediate frequencyzthe center frequency of the intermediate-frequency band determined by the intermediate-frequency selectors.

fo=fsolfio=the oscillator frequency fz=f0ffc=the intermediate carrier frequency.

The graphs of Fig. 2 are plotted similarly to those of Fig. 1, frequency being indicated along the horizontal axis, the ordinates of Figs. 2a and 2c representing relative component frequency intensities, and the ordinates of Figs. 2b and 2d indicating relative gain or responsiveness. The graphs which are like those of Fig. 1 are similarlyV numbered.

Figs. 2a and 2b correspond to Figs. 1a and 1b, respectively, and show the general operation and characteristics of the ordinary superheterodyne double sideband receiver. The oscillator frequency fo is tuned above the signal carrier frequency fsc by the amount of the nominal intermediate frequency fio. The oscillator frequency is represented by the line 22, which corresponds to a dial indication iso, at the receiver, equal to the signal carrier frequency fsc. The signal and oscillator frequencies are combined in the superheterodyne modulator to produce the diiference frequency components represented by graph 23, the converted carrier Wave having its frequency fic equal to the nominal intermediate frequency fio. It is noted that the lower and upper sidebands are interchanged in the process` of converting from signal to intermediate frequency. This is because the signal upper sideband is nearer the oscillator frequency, and therefore produces lower difference frequencies.

'Ihe graph 24 in Fig. 2b, centered at the nominal intermediate frequency fic, shows that the intermediate-frequency selectors respond uniformly over a frequency band which includes both of the intermediate-frequency sidebands. Graph I2 of Fig. 2b shows the responsiveness of the modulator and intermediate-frequency selectorsA in terms of signal frequency, and takes into 'consideration the fact that the latter frequency is to be converted to the intermediate frequency in the modulator. 'Ihe resulting audio-frequency responsiveness, shown by graph I3, is the same as in the case of Fig. 1b.

Figs. 2c and 2d show the operation and characteristics of a superheterodyne single sideband receiver, Fig. 2d corresponding to Fig. ld. The oscillator frequency is tuned lower than in the case of the double sideband reception of Figs. 2a and 2h, by an amount equal to one half of the highest audio frequency required. The oscillator frequency in this case is represented by the line 25 and corresponds to a dial indication fsu Which is likewise lower than the signal carrier frequency fsc. The signal and oscillator frequencies are combined to produce the difference frequency components represented by graph 26. The converted carrier frequency fic is lower than in the case of Figs. 2a and 2b by one half of the frequency fb, and likewise lower than the frequency fio.

The graph 2'I in Fig. 2d shows that the intermediate-frequency selectors respond uniformly over a frequency band which includes only one of the sidebands, graph 2l being half as Wide, in frequency, as graph 24. The graph 21 is centered at the same frequency fm as in the case of double sideband reception illustrated in Fig. 2b, but includes all of only one sideband, as a result of detuning the receiver. In Fig. 2d, graph I 6 shows the responsiveness of the modulator and intermediate-frequency selectors in terms of the signal frequency. The resulting audio-frequency responsiveness, shown by graph II (and I8 when audio-frequency compensation is employed), is the same as in the case of Fig. 1d.

Fig. 2 shows exactly how a superheterodyne receiver would be operated to secure the double sideband and single sideband characteristics of Figs. 1b or 1d, respectively. From that description, it is apparent how to design and operate a superheterodyne receiver with the characteristics of Figs. ld, le, or 1f.

With respect to superheterodyne receivers, the following 'considerations should be appreciated:

(l) The signal carrier frequency fsc is fixed at the transmitter and cannot be varied by adjusting the receiver.

(2) The nominal intermediate frequency f1@ is predetermined by the design of the intermediatefrequency selectors and cannot be varied by adjusting the tuning dial of the receiver. The band width of the intermediate-frequency selectors, shownby graph 24 or 21, is likewise predetermined.

(3) The nominal tuning frequency fsa, indicated on the tuning dial, may be varied at will. The main function of the tuning dial is to vary the oscillator frequency fo, which is normally equal to fsu plus fio. The direct effect of varying theoscillator is to vary the intermediate carrier frequency fic, relative to the nominal intermediate frequency fr0. The indirect effect of varying the oscillator is to vary the position of graph |2 or I6 in Figs. 2b or 2d, which is determined by the oscillator and the intermediate-frequency selectors, and which is centered at fsa, assuming the oscillator frequency fo is always equal to ,fsu plus fm.

(4) The superheterodyne receiver can be regarded as a simple receiver, for most purposes, provided the oscillator frequency fo is always equal to fsu plus fio.

Circuit arrangement and general operation Fig. 3 is a generalized circuit diagram illustrating the arrangement of a superheterodyne type of single sideband receiver in accordance with this invention. The path traversed by signals between the antenna and the loudspeaker is largely conventional. There is provided an antenna 30 and ground 3l for intercepting the signals, which are then supplied in order to a radiofrequency amplifier 32; a local oscillator and modulator 33; intermediate-frequency amplifiers 34 and 35 having interposed therebetween a volume level control 35; a diode rectifier 31; an audio-frequency amplifier 38; and the loudspeaker 39.

With reference to this main signal path, the radio-frequency signal is received in the usual manner by the antenna 3|! and selected and amplified at the signal frequency in the radio-frequency amplifier 32, which for broadcast reception is capable of tuning over the frequency range of about 550-1500 kilocycles. The oscillator and modulator system 33 converts the signal carrier frequency to an intermediate carrier frequency in the manner well understood in the art. The intermediate carrier frequency will be one of two frequencies depending upon which of the two sidebands is being selected. These two alternative intermediate carrier frequencies will therefore differ by the width of a sideband which is about 4 kilocycles. These frequencies may conveniently be 110 or 114 kilocycles, depending upon which sideband is to be utilized. The signal is further amplified in the intermediate-frequency amplifier wherein the carrier and one sideband are selected, the band width passed by this amplifer being, for the particular intermediate carrier frequencies mentioned above, 110-114 kilocycles. The volume level is controlled by the level control 35 interposed between the two intermediate-frequency amplifiers 34 and 36.

The diode rectifier 31 produces from the carrier and the single intermediate-frequency sideband the audio frequencies of modulation, which are then amplified in the usual manner by the audio amplifier 38, from whence they are supplied to the loudspeaker 39. For the purpose of maintaining the volume output of the receiver substantially constant in spite of wide variations of received signal intensity, there is provided an automatic volume control system comprising cunnections 40 from the output terminals of intermediate-frequency. amplifier 34 to the following elements connected in succession: an intermediate-frequency trap 4|, an intermediate-frequency amplifier 42, a diode rectifier 43, and a diode suspender 44. The functions of the intermediate-frequency trap 4| and of the diode suspender 44 are described below. In accordance with the operation of the automatic volume control system, there is created at the diode suspender 44 a uni-directional voltage which varies with the received signal intensity; and this unidirectional voltage is applied through the automatic volume control bias connection 45 to the control elements of amplifiers 32 and 34 and modulator 33. For the purpose of insuring the proper variation of control bias voltage over a wide range of received signal intensities, there is provided a connection 46 from the intermediatefrequency amplifier 34 of the main signal path to a control element of the intermediate-frequency amplifier 42 in the automatic volume control system. 'I'he function and operation of this reverse control bias is more fully described below.

There is also provided a quieting system also operated from connections 40, this quieting system comprisingk in succession a diode rectifier 41 and a direct-current amplifier 48 wherein there is created a direct-current bias which is applied to a control element of mediate-frequency amplifier 36 for maintaining this latter amplifier inoperative until the signal intensity at connections 4B rises above a predetermined value. This inoperativeness of amplifier 36 under the condition of relatively weak signals can be brought about by the use of a high negative bias on its control element. When the signal strength becomes stronger, this quieting bias on the control element is reduced, permitting normal operation. There is also associated with the quieting system a visual tuning indicator 50 connected to the output of amplifier 48, the purpose of the tuning indicator being to enable an operator to readily determine the correct position of the tuning dial.

In accordance with a feature of the invention, there is created what is here termed selective admission, this being effected by the action of the intermediate-frequency trap 4| of the automatic volume control system, and perfected by the cooperation of the quieting system. The trap 4| contains a number of selective circuits proportioned to cause the automatic volume control bias of conductor 45 to become partially relaxed when the receiver is so tuned that the intermediate carrier frequency is located exactly at one edge or the other of the frequency band transmitted by the intermediate frequency amplifier; in this case, 110 or 114 kilocycles. At these tuning points, which represent the proper adjustment of the receiver, the intermediate-frequency carrier voltage present at connections 40 is maintained at a substantially higher level than in the case of any other tuning points. This means that when the receiver is tuned to one or the other of the correct tuning points, the output of amplifier 34 rises abruptly and therefore causes the signal at connections 40 to exceed the critical value above which the quieting action is released. At these correct tuning points, the visual indicator abruptly gives maximum indication to show correct tuning.

Fig. 4 is a detailed circuit diagram illustrating a single sideband receiver designed according to the general arrangement of Fig. 3. The rectangular boxes of Fig. 3 are indicated in Fig. 4 in dotted lines and are similarly numbered.

Before proceeding with a description of Fig. 4, there will be explained some of the conventional notations and symbols used therein. The more important of these symbols are defined with reference to Figs. 4a4f. The tube ||8 illustrates a triode having a cathode 2|, a control grid |22 usually negative relative to the interf cathode, and a plate or anode |23, usually positive relative to the cathode. Tube ||9 is the symbol used for a double-diode triode type of tube, in which the triode elements are cathode |2|, control grid |22, and plate |23, while the plates |24 and |24 are diode plates or anodes. Tube |26 illustrates the symbol used for a pentode tube having a cathode |2|, a control grid |22, a screen |25 usually having a positive voltage relative to the cathode, `a suppressor grid |26, usually tied to the cathode, and a plate |23. The symbol for a variable tuning condenser which is varied while tuning the receiver is shown at |21, While condenser |28 is the symbol for an adjustable (but usually preadjusted) condenser. The battery |29 represents a source of direct voltage, the positive side of which is conventionally indicated by a long line.

Although batteries such as |29 are shown in many places in Fig. 4, it is understood that these represent any source of direct voltage, and the same source may be used simultaneously in several or many places where the symbol |29 is shown. It is also observed that the doublediode triode tube 9 is a multi-purpose tube, that is, the triode elements may be used in one portion of the system and each of the diode elements in different portions. Hence, where such a tube as ||9 is used, it is separately shown at each position in Fig. 4 where any of its elements are used.

In Fig. 4, the radio-frequency amplifier 32 comprises a pentode tube 5| suitably coupled to the antenna 30, and has associated therewith three simultaneously tunable selecting circuits, two of which, designated 64 and 65, are located ahead of the amplifier and the other of which, 66, is coupled between the amplier and the modulator 52.

The oscillator and modulator arrangement 33 comprises the local oscillator tube 53vand the modulator tube 52. The general arrangement of this part of the system is understood in the art and requires no detailed description here. Particular features are described below.

The output of the modulator is the input of the intermediate-frequency amplifier 34 which comprises two amplifying tubes 54 and-55 and the three intermediate-frequency coupling systems |66, |6I, and |62 located before, between and after the amplifying tubes.

The volume level control 35 comprises a movable coil 86 in the input circuit of the following intermediate-frequency amplifier stage 36. The coil 88 is coupled with coil 18 of coupling system 62 and is preferably coaxial therewith. Therefore the signal intensity applied to tube 56 of stage 36 can be` varied at will by axial motion of coil 80.

The signals are rectified in the diode portion of a double-diode triode tube 51 in stage 31. Only one of the diodes is employed for signal detection, this particular diode comprising the cathode |63 and one of the diode anodes |64. The triode elements of this tube are used as the first portion of audio-frequency amplifier 38. For the purpose of illustration, therefore, tube 51 is shown again in the audio amplier 38 where it is designated as tube 51. This second illustration of the same physical elements is proper and convenientlbecause in so far as circuit operation is concerned, it acts as two separate and distinct tubes.

The rectified current iiows through a resistor |65, from whence rectified voltage is impressed upon the control grid of audio amplifier 51 through conductor |66. VThe anode of amplifier 51 is coupled to the second audio-frequency ampliiier 58 through a system of resistances and reactances which include resistance 84 and condenser 85, the function of the latter two elements being to provide proper audio-frequency compensation as will be more fully described hereinafter. The audio signals are further amplified in the push-pull amplifier including tubes 59 and |59, from whence they are conducted to the loudspeaker 39.

Single sideband relations Since it is of primary importance that the receiver select a band of frequencies having a width about equal to the highest required audio frequency of modulation, this selection is provided by the permanently tunedv intermediate-frequency coupling systems of intermediate-frequency amplifier 34. The coupling systems |60, |6|, and |62 each include va double-tuned transformer having syntonous primary and secondary circuits individually tuned to the intermediate frequency, and the coupling is preferably designed to have about the optimum value to provide a uniform transmission characteristic over a band .of frequencies corresponding in width to one sideband. In the particular receiver being described, the band seelcted by the intermediate-frequency selecting system is 110-114 kilocycles. The resulting selectivity is illustrated by the curve |34 of Fig. '7 which is'shown to be flat within two decibles over a band 4 kilocycles wide and centered at the frequency fio equal to 112 kilocycles. The signal-selecting circuits of radio-frequency amplifier 32 cannot be as accurately tuned as the intermediate-frequency coupling systems, and are, therefore, more broadly tuned; hence, the selection is accomplished mainly in the intermediate-frequency amplifier according to the curve of Fig. '1. The responsiveness of the radiofrequency amplifier may and should be kept uniform within about one decibel over the corresponding radio-frequency sideband, and is therefore designed to pass a band about twice as wide.

At the top of Fig. 7, there is shown the frequency spectrum 26 of a desired signal, carrier and both sidebands being shown as indicated by the designations fic, fm', and fib. There are also illustrated the positions of the carrier and both sidebands, |35 and |36, of each of two other signals in adjacent broadcast channels. The curve |34 shows that the receiver is tuned so that the. intermediate-frequency amplifier transmits the upper intermediate-frequency sideband (the lower radio-frequency sideband). For the purpose of this example, it is assumed that signal 35 is stronger than signal |36; hence, the receiver is shown to be properly tuned to the sideband which is furthest away from the strongest signal. The intermediate carrier frequency of signal |35 is subject to 20 decibels more attenuation than that of signal |36, and the nearest sideband frequency of signal |35 is subject to 30 decibels more attenuation than is the nearest sideband frequency of signal |36. Both of the adjacent channel signals are attenuated much more than they would be if the curve |34 were doubled in width, as required for double sideband reception, and it is shown how the receiver may be tuned to discriminate most against the stronger of the signals in the two adjacent channels.

Each of the intermediate-frequency transformers |60, 6| and |62, is provided with two co-axial closed turns of wire 14 and 14 on the side of each transformer coil which is remote from the other transformer coil. These two closed coils are moderately coupled with the corresponding transformer coil, preferably in a coaxial cylindrical shield and serve the purpose of increasing the width of curve |34 to the required value. The desired effect maybe obtained by varying the position of the closed turn or by changing the wire size; and for any given kind of Wire a size is found which has the greatest effect.

Two of the most important problems encountered in the use of this receiver are: (1) to cause the operator to properly tune the receiver so that the intermediate carrier frequency is located at one edge of the selected frequency band, as shown in Fig. '1; and (2) to compensate for the loss of nearly all of the rejected sideband.

The first of these problems is solved by a system of selective admission, which greatly lessens or entirely quiets the response of the receiver when not properly tuned. This system, which has been mentioned above and is described below in more detail,.is made necessary or at least desirable because the sound at the loudspeaker is harsh and unpleasant when the carrier frequency is tuned too far 0E of the edge of the transmitted sideband.

The second ofthese problems is solved by the use of resistor 84 and condenser 85 in the audiofrequency amplifier 38. In Fig. 9, which is plotted as relative gain or responsiveness against audio frequency, curve |40 indicates the equivalent audio-frequency loss resulting from the cutting out of most of one sideband. This loss is shown to be about 3 decibels at one kilocycle and 6 decibels at 4 kilocycles in the case of the particular receiver under discussion. Accordingly, the elements 84 and 85 are proportioned to produce a relative audio-frequency gain which varies substantially as represented by curve |4|. This latter variation being complementary to that of curve |40, uniform overall audio-frequency response is produced as represented by curve |42.

Automatic volume control The proper operation of the quietng system requires that the output of the intermediate-frequency amplifier 34 be maintained at a constant predetermined value, substantially independent of both the received signal intensity an-d the percentage of modulation. In so far as is known, none of the usual systems of automatic volume control meet both of these requirements. The novel type of automatic volume control system used with the present receiver does meet these requirements, and the arrangement and operation of the system is described with reference to Figs. 3, 4, 12, and 13. For the purpose of maintaining the output of amplifier 34 constant across coil 18 in Fig. 4, coil 19 is coupled to coil 18 and connected by connection 40 to the intermediate-frequency trap 4| from where it is amplified in intermediate-frequency amplifier tubes 60 and 6| and then rectified in the rectifier tube 63. The coupling systems 98 and 99, which couple these latter three tubes in succession, are each tuned to a broad band centered at the mid-frequency of the intermediate-frequency band, 112 kilocycles in this case.

Rectifier 63 is shown as being part of a doublediode triode tube, but for rectification only that diode portion comprising cathode |10 and anode |1| is utilized. The remaining.r dodeanole lI'IZ is tied to the cathode so that it is inert, and the grid |13 and plate |14 are utilized as the elements of the triode amplifier 63 associated with the visual tuning indicator in the quieting system, described below.

There is associated with the diode rectifier 63', a bridge circuit formed by resistors |04 and |05 connected in series across direct voltage sources |0|, |02, and |03, the point |51 between these resistances being connected to the lower side of the secondary coil of coupling system 99, and the lower end of resistance 05 being connected to the diode anode |15 of another diode 62, called the suspender diode. The point between voltage sources |02 and |03 is connected to the cathode |10.

The suspender diode 62" isphysically like tube 63', that is, it comprises also a triode having a control grid |16 and a plate |11, these triode elements being utilized in the triode amplifier designated 62 of the quieting system, described below. Although voltage sources |0| and |02 are shown in the diode rectifier section 43 as directly connected in series without any other element connected between them, there is actually a connection from between these two sources to the anode |11 of triode 62.

Each of the diodes 63' and 62" is a uni-directional conductor having, relative to resistors |04 and |05, much less resistance to current in the more conductive direction and much more resistance to current in the opposite direction. Therefore, each of these diodes has an internal resistance, when its anode is positive relative to its cathode, which is much lower than the value of either of the resistances 04 or 05. Therefore, the diode anode |15, which is point |01, is in effect grounded to cathode |18 so long as the junction |51 has a positive voltage relative to ground.

In the absence of a signal, point |51 is maintained l at a positive voltage by voltage sources |0|, |02, and 03, the resulting current flowing through resistors |04 and |05 and diode 62". These resistors and voltage sources are so proportioned that the open circuit voltage at the junction |51 is substantially the same as that at the junction of sources |02 and |03. Consequently, the connection of the cathode |10 and anode 1| of diode 63' between these two junctions of the bridge produces little or no effect in the absence of a signal, because then there is little or no fiow of current through this diode.

When an intermediate-frequency voltage is impressed on diode 63 from coupling system 99, there is produced a rectified current which flows through the bridge and divides between resistors |04 and |05, increasing the current in the former resistor and decreasing the current in the latter resistor and diode 62". Therefore there is superimposed upon the initial voltage of the junction point |51 a rectified voltage having an average value approximately equal to the root-meansquare value of the intermediate-frequency voltage impressed upon this diode. This constitutes a form of linear detection, and the rectified voltage is an undistorted replica of the intermediate-frequency envelope. To produce this result, condenser |00, connected between cathode |10 and junction point |51, is so small that its charging current is negligible at audio frequencies; while condenser |06 connected between point |01 and ground is so large that its audiofrequency impedance is considerably less than that of resistor |05, and therefore the audio-fre- Cil quency voltage at point |01 is negligible at all times regardless of the impedance of diode 62".

Fig. 12 is a chart in which average rectified voltage is plotted against intermediate-frequency carrier voltage, and which shows the approximate relations in the rectifier and suspender circuit as a function of the intermediate-frequency carrier voltage applied to diode 63'. The straight line |46 shows the average voltage of the junction point |51 relative to ground, which voltage changes in the negative direction in proportion to the impressed carrier voltage. It is observed that the total voltage of sources |0|, |02 and |03 is 90 volts, sources |0| and |02 together accounting for 30 volts and source |03 for 60 volts. In the absence of an intermediate-frequency signal, the voltage of point |51 relative to ground is +30 volts, as indicated by the upper end of the graph |46. When an intermediate-frequency voltage of any intensity is applied to the rectifier, the dimension |41 indicates the voltage across resistor |04, and the dimension |48 indicates the voltage across resistor |05. When the signal intensity increases to the point where the voltage of 'junction point |51 becomes negative, the current in diode 62A is reduced to zero, and the average voltage of point |01 then becomes equal to the average voltage of point |51. The voltage ofpoint |01 relative to ground is indicated by dimension |49, and is shown for this example to increase from zero in a negative direction when the intermediate-frequency carrier voltage exceeds 30 volts. The voltage of point |01 is applied through the conductor 45 to the control grids of intermediate-frequency amplifier tubes 54 and 55, and through connection 45 to the control grid of modulator 52 and of radio-frequency amplier 5|. The action of such automatic volume control biasing connections for controlling the gain of amplifiers and modulators is well understood in the art and requires no detailed discussion here. It is sufficient to state that the connection 45 causes the control grids of the controlled tubes to become more negative when the signal strength increases above a predetermined value, thereby maintaining the output level fairly uniform.

Changes in the degree of modulation of the received signal are prevented from affecting the automatic volume control bias in the following manner. The rectified voltage at point |51 is an undistorted replica of the intermediate-frequency modulation envelope, and therefore the average rectified voltage is equal to the rectified intermediate-frequency carrier voltage, for any degree of modulation up to 100%. Since the audio modulation frequency components of rectification are filtered out by condenser |06, the `automatic volume control bias and the effect of diode 62 depends only upon the intermediate-frequency carrier voltage. This type of suspended automatic volume control utilizing a suspender diode 62", which fails to exert anyv volume controlling influence until the carrier voltage exceeds a predetermined value, is a preferred method of performing this result. This advantage plus freedom from modulation effects is not obtained in other systems now in common use.

In the particular receiver under discussion, 'it is desired that the amplification' be capable of 100 decibels variation by the automatic volume control action, in order that received signals varying in intensity from 10 micro-volts' to 1 volt may produce a uniform output.` This requiresV that the bias represented by the dimension |49 in Fig. 12, applied to the control grids of tubes' 5|, 52, 54, and 55, shall vary from zero to about 30 volts negative relative to ground, corresponding to the above minimum and maximum received signal voltages. The production of this wide variation of control bias requires that the intermediate carrier frequency voltage applied to diode 63 shall vary from about 30 to 60 volts over the same range of received signal intensity, smaller intensities producing no volume control bias. These relationships are shown in Fig. 12.

The desired highly uniform output of intermediate-frequency amplifier 34 is accomplished by means of a reverse automatic volume control bias voltage provided by a connection 46 extending from the cathode of amplifier tube 55 to the cathode of amplifier tube 60 of the automatic volume control system. This common cathode connection 46 is connected to ground through a resistance including rheostat 16 which carries the space current of both tubes; hence, the space current of tube 55 partially controls the grid-cathode bias of tube 60, and therefore the gain of this tube. This bias on tube 60, however, varies oppositely from that on the controlled amplier and modulator tubes of the main signal path. When the grid-cathode bias of arnplifier 55 becomes greater by automatic volume control action, its amplification and space current decrease, and consequently the grid-cathode bias 0n tube 60 decreases, causing the gain of the latter tube to increase somewhat. Therefore, the increased intermediate-frequency voltage, which is required by diode 63' to decrease the amplification of amplifier 34, is supplied by the increased gain of tube 60 and without any increase of the input to the latter tube, or of the output of amplifier 34. This action is called reverse automatic volume control because the gain of tube 60 is automatically varied oppositely to that of tubes 5|, 52, 54, and 55. Quantitatively, the gain of tube 60 can be just about doubled by this reverse automatic volume control connection, this being sufficient to vary the intermediate-frequency voltage on diode 63' over the above-mentioned range of 30 to 60 volts without any appreciable variation of the intermediate-frequency input to tube 60.

The foregoing discussion describes the operation of the reverse and the suspended automatic volume control actions. These cooperate to maintain the output of intermediate-frequency amplifier 34 very nearly uniform, within plus or minus l decibel, during received signal intensity variations up to 100 decibels, this representing approximately a 99% approach to perfect automatic volume control. For the particular quantitative example described, the resulting automatic volume control action isillustrated by curve |50 of Fig. 13 which shows the relative audio-frequency output in decibels plotted against the received radio-frequency input voltage in microvolts.

An adjustment of the output level of amplifier 34'is furnished by the rheostat 16, Whose function is to control the gain of tube 60 and thereby to determine the output level of amplifier 34 which is required to operate the automatic volume control system. The adjusting rheostat 16 is preferablypreadjusted to the proper value and not changed during operation of the receiver. Its proper setting is mainly determined by the action of the quieting circuit described below in detail.

Ll i) plifier tube 54, this constituting a manual variation of the grid bias of the tube which, therefore, controls its gain, and is able to make it less sensitive to noise and to weak signals too noisy to be useful. When the rheostat is adjusted for minimum sensitivity of the amplifier, the automatic volume control characteristic of the receiver is modified somewhat as shown at curve |50' in Fig. 13, which shows the change of audiofrequency output with radio-frequency input. Incidentally, it is desired that the sensitivity control 15 be mechanically connected, as indicated by broken line |19, with a switch ||2 associated with amplifier 62 of the quieting system so that the latter switch is automatically opened when the control 15 is adjusted to or beyond maximum sensitivity, thereby rendering the quieting circuit inoperative and placing the receiver in condition to respond to all signals and noises however weak.

Selective admission The selective admission, mentioned above, is provided for causing the operator to correctly tune the receiver for single sideband reception. This includes means for causing the audio-frequency output to sharply reach a maximum value when the intermediate carrier frequency is tuned on either edge of the band selected in the intermediate-frequency amplifier 34. This action is an effect of the frequency-discriminating circuits of trap 4| in cooperation with the other sections 42, 43, and 44 of the automatic volume control channel, and is perfected by the cooperation of the quieting circuit.

Curve |45 of Fig. 11 shows the relative gain or responsiveness of the intermediate-frequency trap or discriminating network 4|, the ordinates representing relative gain in decibels and the abscissas representing the frequencies in and near the selected intermediate-frequency band. Curve |45 then represents the relative gain from the output coil 18 of ampliiier tube 55 to the grid of tube 60, the amplifier output being applied to the trap through the connection 40 from coil 19 which is coupled to coil 18. In the trap network, coil 92 and condenser 94 are sharply tuned to the upper edge of the selected sideband (fi0-|-fb/2=112+2=114 kilocycles, in this case) and produce the right minimum of the trap curve |45. Coil 93 and condenser 95 are equally sharply tuned to the lower edge of the sideband (fi0-fb/2=112-2=110 kilocycles, in this case) and produce the left minimum of the trap curve. The four elements 92, 93, 94, and 95, taken together, form a parallel circuit resonant at the mid-frequency fio of the band (112 kilocycles in this case) and serve to produce the center maximum at frequency fio. Coils 19 and 96 and condenser 91, in parallel, are likewise resonant at the frequency ,fio and cooperate with the other four elements of the trap to produce the higher maxima on either side of the band.

In this arrangement, coil 19 is only moderately coupled to coil 18 so as not to materially affect the characteristics of the latter in the tuned intermediate-frequency circuit. The loss of voltage in the trap circuits is more than made up by the gain in amplifier 42. The trap circuits are preferably enclosed in a shield, and the junctions of elements 92 and 94 and of elements 93 and 95 enclosed therewithin, thus preventing accidental coupling to these elements. These elements should also be shielded from each other. A particular advantage of this arrangement is that the grid-cathode capacity of tube 60, or any other outside circuit properties, do not in any way affect the frequencies of minimum gain of the trap.

The effect of the selective admission feature upon the output of intermediate-frequency amplifier 34 is illustrated by curve |52 of Fig. 14 and curve |55 of Fig. 15, these latter two figures having negative rectified intermediate-frequency voltage plotted as ordinates and frequency as the abscissas. Curves |52 and |55 are inversely proportional to curve |45 in Fig. 11. They represent the effect of the trap in making the automatic volume control action dependent on the intermediate carrier frequency and therefore on the tuning of the receiver. Curve |52 of Fig. 14 represents the rectified voltage from diode 62 of the quieting channel, due to the connection 40" from amplifier 34 to the quieting channel. For

a given setting of the volume level control 80,-

curve |55 of Fig. 15 shows the variation of the rectified Voltage at diode rectifier 51', due to the intermediate-frequency voltage impressed thereon from amplifier 56. In Fig. 15, the ordinates are proportional to the audio-frequency sound from the receiver.

Automatic quieting The automatic quieting system proper cornprises connection 40", tubes 62 and 62, and the quieting connections 49 and 49. It is the means for quieting the receiver at all times except when the receiver is properly tuned to receive a signal above the noise level. This system quiets undesired noises and distorted signals, which would otherwise be produced when the receiver is not properly tuned to a useful signal. As used in this receiver, the quieting system cooperates with the trap 4| and the automatic volume control system to perfect the selective admission.

The quieting action is performed by automatically varying the bias between control grid and cathode of intermediate-frequency amplifier tube 56, this bias being maintained so highly negative under the condition of no signals or too weak signals that this tube is substantially inoperative. When the signal strength rises above a predetermined threshold value, the connection 49 restores the bias to its normal operating value and tube 56 is made operative. This bias is determined by the voltage of source |02, and by the additional voltage which supplies the normal bias voltage, produced across resistor ||3 by the plate current of triode 62, assuming switch 2 is closed. When this plate current is considerable, the grid bias of tube 56 is sucient to reduce its gain to zero.

In operation, the intermediate-frequency voltage at connection 40" is rectied by diode 62', which is of the linear type producing an undistorted replica of the modulation envelope. The audio-frequency modulation components of this rectification are filtered out by a resistor I0 and condenser connected in the order recited between the anode |80 and ground, and the uni-directional rectified component, proportional to carrier but independent of modulation, is applied to the grid |16 of triode 62 from the junction of resistor ||0 and condenser As shown above, the plate current of triode 62 serves, in the absence of a signal, to apply an excessive bias on tube 59, thereby quieting the receiver. But when the amplified intermediate carrier frequency voltage on connection 40 increases. the rectified current of rectifier 62' increases, the grid |16 is made more negative, and the plate current of tube 62 flowing through resistor I3 is reduced. When the amplified carrier voltage exceeds a predetermined value, the plate current of tube 62 is reduced substantially to zero and the bias of amplifier 56 is thereby reduced substantially to its normal value for operation as an amplifier.

The quieting of noise and weak signals is illustrated in Fig. 13 by curves |5| and |5|', which are respectively like curves |56 and |56 except for the dotted portions. Curves |5| and |5| show that when the signal input falls below a given threshold value, the receiver is quieted by reducing the audio-frequency output to zero. The threshold value of the signal input voltage is determined by the setting of rheostat l5, as previously mentioned. When the quieting system is disabled by setting rheostat 'l5 for minimum resistance and thereby opening switch H2, the operation of the receiver is characterized by curve |56, Curve |56 can be obtained by operating switch ||2 independently when rheostat 15 is set for high resistance.

The quieting system cooperates with the trap l 4| to perfect the selective admission, during the reception of any signal strong enough to operate the automatic volume control system, in the manner shown graphically in Figs. 14 and 15. In Fig. 14, curve |52 shows the variation of the rectified intermediate-frequency voltage in diode 62', as a function of the intermediate carrier frequency. Curve |53 shows the correspo-nding variation of the negative grid-bias voltage applied to the grid of tube 56 through connection 49, this curve being similar in shape to the upper part of curve |45 in Fig. 11. Curve |54 illustrates the corresponding variation of the plate current of tube 56, showing that space current flows only when the negative grid bias impressed by connection 49 drops to a low value. This illustrates the quieting action due to this tube. The selective admission without quieting produces at the diode 57 a variation of rectified intermediate carrier frequency voltage shown by curve |55 of Fig. 15. When the quieting action is cooperating, the rectified voltage at diode 51 varies as shown in curve |56.

When the operator tunes the receiver, he varies the intermediate carrier and sideband frequencies. Figs. 11, 14, and 15 therefore show the operation of the quieting and selective admission while tuning the receiver. Curve |56, in particular, shows that the signal is heard only when the receiver is tuned very close to one of the two points where the intermediate carrier frequency is on o-ne edge of the band selected according to curve |34 of Fig. '7.

When a weak distant station is being tuned in at the same time that a strong local station is transmitting on an adjacent channel, the latter operates to quiet the receiver when the receiver is tuned to the sideband of the distant signal which is nearer the local signal channel. This action is desirable, because under such circumstances the distant signal can be heard free from interference only when tuned to the sideband further from the local signal channel.

Visual tuning indicator There is connected to the plate of tube 62 the grid |'i3 of triode tube 63 in the direct current amplifier 48. In the space current path of tube C3 is a visual tuning indicator in the form of a low current incandescent lamp 56. There is applied through the lamp an initial current through resistor from direct voltage source ||6, sufcient to heat the lamp filament to the point of incipient incandescence when no space current is flowing in tube 63. The grid bias of triode 63 varies in the same manner as that of tube 56, so that no space current ows in tube 63 until the signal strength reaches a predetermined value. The space current of tube 63 fluctuates simultaneously with that of tube 56 (since both are connected to the plate of tube 62) and therefo-re the relative lamp brilliancy is indicated by curve |54 of Fig. 14. The condition of greatest brilliance indicates the correct tuning position. The switch ||2 must be closed to render the tuning indicator operative.

The incandescent filament lamp is satisfactory, since the variation of its current during the tuning operation may be several milliamperes. A neon or other type of lamp can be used satisfactorily for the same purpose with good results.

Time lag relations Good operation of the receiver requires that the automatic volume control should operate in a time comparable with the period of the lowest audio-frequency modulation to be reproduced. This time lag of automatic volume control operation is determined mainly by resistor |65 and condenser` |66, the time constant of which may well be 1/40th second.

The time lag of operation of the quieting system should be at least as great, and preferably greater, than that of the automatic volume co-ntrol, so that while a signal is being tuned in, the receiver will remain quiet until the automatic volume control has had time to operate. The time lag of the quieting system is determined mainly by resistance I4 and condenser H5, and secondarily by resistance ||6 and condenser The time constant of the former two elements may well be the same as that of the automatic volume control, 1/40th second, and the time constant of the latter two elements may well be 1/ 100th second. The total time constant of the quieting system, as thus proportioned, will therefore be .035 second which is 46% greater than that of the automatic volume control.

The lamp 56 has sufcient time lag due to thermal inertia, so that the electrical time lag is undesired. The electrical time lag is reduced by connecting the grid of triode 62 ahead of elements ||4 and ||5, so that the total electrical time constant for the lamp is that due to elements ||6 and or 1/100th second.

Volume level control Referring now to the volume level control, coils 18, 19, and 86 are preferably located coaxially within a non-magnetic metallic cylindrical shield. Angular rotation of the control knob imparts axial displacement to coil 86, and therefore it is found that the mutual inductance between coils 18 and 86 varies exponentially with the displacement. Plotted in terms of decibels variation of output against angular displacement, this relation is represented by the straight line |31 of Fig. 8, this being the ideal relation between displacement and audio-frequency output. Curve |36 shows the corresponding curve of variation for a linear (instead of exponential) potentiometer, and curve |39 shows the corresponding characteristic of a linear potentiometer having a high resistance half and a low resistance half. The

smoother and superior operation of the exponential type of variation is apparent.

The potentiometer 86 is controlled simultaneously by the same knob, as indicated by broken line |8|, which controls the volume level control 35. This potentiometer serves to modify the audio-frequency fidelity simultaneously with volume level. Experience teaches that the normal ear desires less change of intensity at the higher and lower audio frequencies than at the medium audio frequencies. The delitycontrol is designed to satisfy this desire of the ear. At the greatest volume level, the contact of potentiometer 86 is situated at the upper end thereof. In this position, the elements are proportioned so that the delity is according to curve |43 of Fig. 10, which is plotted in terms of relative gain against audio frequencies. At the lowest volume level, the contact is at the lower end, and in this position, condenser 90 causes the fidelity curve to slope upward at the lower frequencies as shown by curve |44. The condenser 8 and resistor 89 at the same time cause the curve to slope upward at the higher frequencies, as shown. At mean volume level, the fidelity is according to curve |42, which is substantially level.

Oscillator and modulator The tuning condenser of the tuned circuit 6l of oscillator tube 53 is like those of the radiofrequency selector circuits 64, 65, and 66, all being on the same shaft as indicated by broken lines |82, and all having the same variation of capacitance. The oscillator frequency is required to be always greater than that of the radio-frequency selective circuits by a constant amount, which is 112 kilocycles in this case. This is ordinarily accomplished by adjusting three properties of the oscillator tuned circuit to be different from the selective tuned circuits as follows:

(a) A relatively large adjustable condenser |90 is added in series with the oscillator tuned circuit;

(b) The oscillator tuned inductance |9| is made somewhat smaller than those of the signal selective circuits;

(c) The minimum value of the oscillator tuning condenser is adjusted to be slightly larger than that of the other tuning condensers.

The above three degrees of freedom permit adjustment of the oscillator alignment to be exactly correct at three points throughout the tuning range, but the errors at other frequencies in the range may be substantial. This is shown by curve 30 of Fig. 5, in which oscillator error is plotted against tuning frequency. The points Where the curve crosses the horizontal zero axis are the three points made correct by the three adjustments.

The alignment error can be further reduced by adding a fourth degree of freedom, which is:

(d) One or more resistors 68 and 69 may be connected across coils fairly closely coupled to the oscillator tuned circuit.

The resulting errors of alignment when the fourth degree of freedom is employed are as indicated by curve |3| of Fig. 5, which crosses the zero axis at four points and shows much less error. The above degrees of freedom have been named in the order of decreasing effect at lower frequencies and increasing effect at higher frequencies. The last is most effective at the high end of the tuning range.

The cathode conductance of the modulator 52 is substantially equal to its transconductance sm, and is located across resistor 68. Since the transconductance varies with the automatic volume control bias on the control grid of the modulator, the volume control system has some undesired effect upon the oscillator frequency. It is desired to minimize this undesired effect without impairing the modulator performance.

Curve |32 of Fig. 6 shows the transconductance sm of modulator 52 as a function of the voltage on its screen |95, in the case of a small initial negative bias on control grid |94. While the transconductance determines the straight amplication of the tube, its modulation is determined by where eg represents the grid-cathode bias. The variation of the quantity sm' with screen voltage is shown by curve |33.

In the case under consideration, it is observed that reducing the screen Voltage from to 40 volts does not appreciably affect the value of sm but does reduce the transconductance sm in the ratio of 5:1. This fact is of great advantage in minimizing changes of oscillator frequency. It also has the advantage of reducing the straight intermediate-frequency gain of the tube, such gain not being desired in a modulator, as it serves to increase noise and interference.

The use of a xed low screen voltage is not desirable, because the grid-bias control would become less gradual and thereby increase distortion in the modulator. The best result is obtained by the use of resistor and by-pass capacity 12 associated with the screen |94. The resistor reduces the screen voltage at low values of grid bias but causes the screen voltage to resume the normal rated value when the screen current is decreased by the presence of greater automatic volume control bias. As a result of this arrangement, there are obtained all the benefits of lower screen voltage when the grid bias is at its minimum value, and also the benefit of higher screen voltage when the grid bias is greatly negative.

I claim:

1. In a modulated carrier frequency signal receiver including a selector uniformly responsive only within a narrow frequency band, tuning means for varying the frequency difference between the carrier and the middle of said band, indicating means responsive to the carrier, and a selective network coupled to the output of said selector and associated with said indicating means for causing the latter to give a critical predetermined indication only when said frequency difference is half the width of said band.

2. In a modulated carrier frequency signal receiver including a selector uniformly responsive only within a narrow frequency band, tuning means for varying the frequency difference between the carrier and the middle of said band, visual indicating means responsive to the carrier, and a selective network coupled to the output of said selector and associated with said indicating means for causing the latter to give a critical predetermined visual indication only when said difference is half the width of said band.

3. In a modulated carrier frequency signal receiver including a selector uniformly responsive only within a narrow frequency band, tuning means for varying the frequency difference between the carrier and the middle of said band, auxiliary means responsive to the carrier for automatically determining the signal output of the receiver, and a selective network coupled to the output of said selector and associated with said auxiliary means for causing the latter to produce maximum signal output when said frequency difference is half the width of said band.

4.. In a modulated carrier frequency signal receiver including a selector-uniformly responsive only within a narrow frequency band, tuning means for varying the frequency difference between the carrier and the middle of said band, auxiliary means responsive to the carrier and unresponsive to modulation for automatically determining the signal output of the receiver, and a selective network coupled to the output of said selector and associated with said auxiliary means for causing the latter to produce maximum signal output when frequency difference is half the width of said band.

5. In a modulated carrier frequency signal receiver including a selector uniformly responsive only within a narrow frequency band, tuning means for varying the frequency difference between the carrier and the middle of said band, auxiliary means responsive to the carrier for automatically determining the carrier output of the receiver, and a selective network coupled to the output of said selector and associated with said auxiliary means for causing the latter to produce maximum carrier output when said difference is half the width of said band.

6. In a modulated carrier frequency signal receiver including a selector uniformly responsive only within a frequency band sufficiently wide to include the carrier and one sideband but too narrow to include both sidebands, tuning means for varying the frequency difference between the carrier and the middle of said band, auxiliary means responsive to the carrier for automatically determining the signal output of the receiver, and a selective network coupled to the output of said selector and associated with said auxiliary means for causing the latter to produce maximum signal output when said frequency difference is half the width of said band.

'7. In a modulated carrier frequency signal receiver including a selector uniformly responsive only within a narrow frequency band, tuning means for adjusting the carrier frequency, auxiary means responsive to the carrier for automatically determining the signal output of the receiver, and a selective network coupled to the output of said selector and associated with said auxiliary means for causing the latter to produce maximum signal output when the carrier frequency is adjusted to one edge of said band.

8. In a modulated carrier frequency signal receiver including a selector uniformly responsive only within a narrow frequency band, tuning means for adjusting the carrier frequency, and auxiliary means responsive to the carrier for producing maximum signal output only when the carrier frequency is adjusted to either edge of said band.

9. In a modulated carrier frequency signal receiver including a tunable selector unifo-rmly responsive only within a narrow frequency band, auxiliary means responsive to the carrier for automatically determining the signal output of the receiver, and a selective network coupled to the output of said selector and associated with said auxiliary means for causing the latter to produce maximum signal output only when one edge of said band is tuned to the carrier.

l0. In a modulated carrier frequency signal receiver including a selector uniformly responsive only within a narrow frequency band, means for adjusting the carrier frequency, and auxiliary means responsive to the carrier for producing maximum signal output only when the carrier 2 11. In a modulated carrier frequency signal receiver, signal selecting means for uniformly selecting the carrier together with all of one sideband and only the inner edge of the other sideband, means for rectifying the selected signal to produce the modulation frequencies, means for amplifying the lower modulation frequencies for which both sidebands are selected, and means for doubly amplifying the higher modulation frequencies for which only one sideband is selected, both said means for amplifying being embodied in a common amplifier including in one stage in series a coupling impedance having resistance and capacitance in parallel for doubling the amplication of the higher modulation frequencies.

12. In a modulated carrier frequency signal receiver including a frequency band selector and an amplifier, tuning means for varying the frequency difference between the carrier and the middle of said band, auxiliary means for automatically reducing the amplifier gain in proportion to increasing carrier intensity and a selective network for coupling the output of the ampliiier to the auxiliary means, said network being proportioned to minimize the action of the gain reducing means when said frequency difference has a predetermined substantial value not exceeding half the width of said band.

tuned to either edge of said band.

13. In a modulated carrier frequency signal receiver including a frequency band selector and an amplier, tuning means for varying the frequency difference between the carrier and the middle of said band, auxiliary means for automatically reducing the amplifier gain in proportion to increasing carrier intensity, and a selective network for coupling they output of the ampliiier to the auxiliary means, said network being proportioned to magnify the action of the gain reducing means when said frequency difference has a value substantially exceeding half the width of said band.

14. In a modulated carrier frequency signal receiver including a frequency band selector and an amplifier, tuning means for varying the frequency difference between the carrier and the middle of said band, auxiliary means for automatically reducing the amplifier gain in propor tion to increasing carrier intensity, and a trap circuit sharply tuned to a frequency in said band but substantially displaced from the mid-frequency thereof for coupling the output of the amplifier to the auxiliary means, whereby the action of the gain reducing means is minimized when the carrier frequency is equal to the trap frequency.

l5. In a. modulated carrier frequency signal receiver including a frequency band selector and an amplifier, tuning means for varying the frequency difference between the carrier and the middle of said band, auxiliary means for automatically reducing the ampliiier gain in proportion to increasing carrier intensity, and a trap circuit sharply tuned to one edge of said band for coupling the output of the amplifier to the auxiliary means, whereby the action of the gain reducing means is minimized when said frequency difference has a value equal to half the width of said band.

16. In a modulated carrier frequency signal Cir receiver including a frequency band selector and an amplifier, tuning means for Varying the frequency difference between the carrier and the middle of said band, auxiliary means for automatically reducing the amplifier gain in pro:

is resonant at one edge of said band and the other of which is resonant at the other edge of said band, whereby the signal output of said amplifier is caused to be greatest when said car- ,rier frequency is located at either edge of said portion to increasing carrier intensity, and akband. double trap circuit sharply tuned to both edgesf \21. In a modulated carrier frequency signal of said band for coupling the output of the amplier to the auxiliary means, whereby the action of the gain reducing means is minimized when said frequency difference has a value equal to half the width of said band.

17. In a modulated carrier frequency signal receiver, an amplifier, a selective system for selecting a band of frequencies which includes a carrier frequency and an associated modulation sideband to be translated, an automatic volume control system responsive to the output of said amplifier for maintaining said amplifier output relatively constant in spite of wide changes in the intensity of the signal input to said amplifier, and a discriminating network interposed between the amplifier output terminals and said automatic volume control system which is least responsive at an edge of said band of frequencies, whereby the signal output of said amplifier is caused to be greatest when said carrier frequency is located at said edge of said band.

18. In a modulated carrier frequency signal receiver, an amplifier, a selecting system for selecting a band of frequencies which includes a carrier frequency and an associated modulation sideband to be translated, an automatic volume control system responsive to the output of said amplifier for maintaining said amplifier output relatively constant in spite of wide changes in the intensity of the signal input to said amplifier, and a discriminating network interposed between the amplifier output terminals and said automatic Volume control system which is least responsive at each edge of said band of frequencies, whereby the signal output of said amplifier' is caused to be greatest when said carrier frequency is located at either edge of said band.

19. In a modulated carrier frequency signal receiver,- an amplifier, a selecting system for selecting a band of frequencies which includes a carrier frequency and an associated modulation sideband to be translated, an automatic volume control system responsive to the output of said amplifier for maintaining said amplifier output relatively constant in spite of wide changes in the intensity of the signal input to said amplifier, and a discriminating network interposed between the amplifier output terminals and said automatic volume control system which is least responsive at both edges of said band of frequencies and is most responsive at frequencies between and beyond said edges, whereby the signal output of said amplifier is caused to be greatest when said carrier frequency is located at either edge of said band.

20,1n a modulated carrier frequency signal receiver, an amplifier, a selecting system for selecting a band of frequencies which includes a carrier frequency and an associated modulation sideband to be translated, an automatic volume control system responsive to the output of said amplifier for maintaining said amplifier output relatively constant in spite of wide changes in the intensity of the signal input to said am plifier, and a discriminating network interposed between the amplifier output terminals and said automatic volume control system, said network having two resonant trap circuits. one of which receiver, an amplifier, a selecting system for selecting a band of frequencies which includes a carrier frequency and an associated modulation sideband to be translated, an automatic volume control system responsive to the output of said amplifier for maintaining said amplifier output relatively constant in spite of wide changes in the intensity of the signal input to said amplifier, a quieting system for rendering a portion of said translating system inoperative when the amplifier output is below a predetermined level, and a discriminating trap associated with said automatic volume control system which is least responsive at the frequency of an edge of said frequency band, whereby the signal output of said amplifier is caused to be greatest when said carrier frequency is located at an edge of said band, and is caused to decrease sufficiently to actuate the quieting system when said carrier frequency is not so located.

22. In a modulated carrier frequency signal receiver including a carrier amplifier, auxiliary means excited by the carrier output of said amplifier for producing a unidirectional voltage tending to vary directly with the carrier output amplitude, means for utilizing said voltage to vary inversely with carrier input amplitude the gain of said amplifier, and means for simultaneously varying directly with the carrier input amplitude the unidirectional voltage produced in said auxiliary means by an unvarying value of said carrier output applied thereto, all of said means being relatively proportioned automatically to maintain the carrier output of said amplifier substantially constant.

23. In a modulated carrier frequency signal receiver including a carrier amplifier, auxiliary means for further amplifying and then rectifying the carrier output of said amplifier, and means for utilizing the rectified carrier to reduce the gain of said amplifier and simultaneously to increase said ampliiication in said auxiliary means, both said means being relatively proportioned to automatically maintain the carrier output of said amplifier substantially constant.

24. In a modulated carrier frequency signal receiver including a carrier amplifier having a vacuum tube, auxiliary means including a carrier rectifier and also a vacuum tube for coupling the carrier output of said amplifier to said rectifier, means for utilizing the rectified carrier voltage from said rectifier negatively to bias said vacuum tube and thereby to reduce the gain in said amplifier with increasing carrier amplitude, and means including a connection between the cathodes of both said vacuum tubes and a resistor common to the circuits of said cathodes, said latter means and said resistor being relatively proportioned automatically to maintain the carrier output of said amplifier substantially constant.

25. In a modulated carrier frequency signal receiver, a first carrier amplifier for repeating signals to detecting apparatus, means for automatically controlling the carri-er amplification in said amplifier, comprising a second carrier amplifier, a rectifier responsive to the carrier output of said second amplifier whereby said rectifier develops a unidirectional voltage increasing with carrier amplitude, a connection from an output electrode of said rectifier to a control electrode of the first amplifier for biasing said control element negatively with said unidirectional voltage, whereby the amplification in said first amplifier is decreased with increasing carrier amplitude, and a connection from a point of said first amplifier to a point of said second amplifier for causing the effective bias voltage on a control electrode of said second amplifier to decrease with increasing bias on said control element of said first amplifier, said bias voltages being proportioned to maintain the carrier output of said first amplier substantially constant.

26. In a modulated carrier frequency signal receiver including means for quieting the receiver when the carrier amplitude is less than a threshold value,the arrangement for adjusting the quieting effect which comprises adjustable means for controlling the threshold value by adjusting the gain of the receiver, and a switch linked to said adjustable means for cutting out the quieting action only when the gain of the receiver is adjusted to maximum.

27. In a modulated carrier frequency signal receiver, a carrier frequency adjustable volume control circuit, a rectifier, a vacuum tube for coupling the volume control circuit output to said rectifier, and means for automatically reducing the translation ratio of said tube when the input to said volume control circuit is less than a predetermined threshold value, whereby disturbances below said threshold value are subject to reduction prior to rectification in said rectifier.

28. In a modulated carrier frequency signal receiver, means for tuning said receiver to a desired carrier frequency, means for quieting said receiver when the desired carrier amplitude is less than a predetermined threshold Value, and selective means most responsive to interfering signals on slightly different frequencies, for operating the quieting means when the ratio of interfering signal to desired carrier amplitude exceeds a predetermined value.

29. In a modulated carrier frequency signal receiver including a carrier amplifier, means for tuning said receiver to a desired carrier frequency, selective means for automatically reducing the amplifier gain with increasing sum of the amplified desired carrier amplitude plus a multiple of the amplitude of any amplified interfering signals on slightly different frequencies, and means for' automatically quieting said receiver when the sum of the amplitudes of all amplified signals is less than a predetermined threshold value, whereby the system is quieted when the amplitude ratio of interfering signal to desired carrier exceeds a predetermined value.

30. In a modulated carrier frequency signal receiver, means for automatically quieting the system when the carrier amplitude is less than a predetermined threshold value, means for delaying the quieting action to avoid its responding to impulses of short duration, and means for visually indicating with less delay the condition of the quieting means at all times.

3l. In a modulated carrier frequency signal receiver including a carrier amplifier, means for automatically reducing the amplifier gain with increasing carrier amplitude, means for quieting the receiver when the carrier amplitude is less than a predetermined threshold value, means for delaying the gain reducing action to avoid its responding to low frequency modulation,

means for delaying to a greater degree the quieting action, and means for visually indicating without substantial delay the condition of the quieting means at all times.

32. In a modulated carrier frequency signal receiver including a selector uniformly responsive only within a narrow frequency band, tuning means for varying the frequency difference between the carrier and the middle of said band, indicating means responsive to the carrier, and a selective network less responsive at the edge frequencies of said band than at the intermediate frequencies of said band and all frequencies without said band, said network being coupled to the output of said selector and being associated with said indicating means for causing the latter to give a clear indication when said frequency difference is half the width of said band.

33. In a modulated carrier frequency signal receiver including a selector uniformly responsive only within a narrow frequency band, tuning means for varying the frequency difference between the carrier and the middle of Said band, indicating means responsive to the carrier, and a selectiv-e network less responsive at an edge frequency of said band than at the intermediate frequencies of said band and of frequencies witho-ut said band, said network being coupled to the output of said modulator and being associated with said indicating means for causing the latter to give a clear indication when said frequency difference is half the width of said band.

34. In a modulated carrier frequency signal receiver including a selector uniformly responsive only within a narrow frequency band, tuning means for varying the frequency difference between the carrier and the middle of said band, visual indicating means responsive to the carrier, and a selective network less responsive at the edge frequencies of said band than at the intermediate frequencies of said band and all frequencies without said band, said network being coupled to the output of said selector and being associated with said indicating means for causing the latter to give a clear indication when said frequency difference is half the width of said band.

35. In a modulated carrier frequency signal receiver including a selector uniformly responsive only within a narrow frequency band, tuning means for varying the frequency difference between the carrier and the middle of said band, visual indicating means responsive to the carrier, and a selective network less responsive at an edge frequency of said band than at the intermediate frequencies of said band and all frequencies without said band, said network being coupled to the output of said selector and being associated with said indicating means for causing the latter to give a visual indication when said frequency difference is half the width of said band.

36. In a modulated carrier frequency signal receiver, signal selecting means for uniformly selecting the carrier together with all of one sideband and only the inner edge of the other Sideband, a diode rectifier circuit for producing linear rectification of a strongly modulated selected signal to produce the modulation frequencies, said rectifier having the property of distortionless rectification of carrier and sidebands when both sidebands are present and their sum is less than the carrier, and also when only one sideband is present and does not greatly exceed half the carrier.

37. In a modulated carrier signal receiver including a carrier amplifier, auxiliary means responsive to the carrier output of said amplifier for producing a unidirectional voltage increasing with said carrier output, means utilizing said voltage for reducing the gain of said amplifier with increasing carrier output thereof, and means responsive to the carrier input to said amplifier for simultaneously increasing with increase in said carrier input the uni-directional voltage produced in said auxiliary means in response to a given value of said carrier output, all of said means being relatively proportioned to vary automatically the gain of said amplifier in the inverse ratio of said carrier input without appreciable variation of said carrier output for a wide variation of said carrier input.

38. In a modulated carrier frequency signal receiver including a carrier amplifier, means for tuning said receiver to a desired carrier frequency, means for automatically controlling the amplifier gain, selective means couplingthe output of said amplifier to said gain control means and including a trap circuit proportioned to cause said gain control means to reduce said amplifier gain with increasing sum of the amplified desired carrier amplitude plus a multiple of the amplitude of any amplified interfering signals on slightly different frequencies, and means for automatically quieting said receiver when the sum of the amplitude of all amplified signals is less than a predetermined threshold value, whereby the system is quieted when the amplitude ratio of the interfering signal to the desired carrier exceeds a predetermined value.

39. In a modulated carrier frequency signal receiver, a band selector for selecting a band of frequencies including a desired signal, an amplifier, auxiliary means for automatically reducing the amplifier gain in response to the signal output thereof, and selective means substantially more responsive at frequencies outside said band than at the desired signal frequency, said selective means coupling the output of said amplifier to said auxiliary means and magnifying the action of the gain reducing means toward any interfering signal on said frequencies outside said band relative to the action thereof toward said desired signal, whereby the amplifier gain is reduced in response to such interfering signals t a greater extent than in response to desired signals of the same order of intensity.

40. In a modulated-carrier frequency signal receiver including a selector uniformly responsive only within a narrow frequency band, tuning means for varying the frequency difference between the carrier of a desired signal and the middle of said band, indicating means, and a selective network more sharply tuned than said selector and controlling said indicating means as a function of said frequency difference for causing the latter to give a critical predetermined indication only when said frequency difference is half the width of said band.

41. In a modulated-carrier frequency signal receiver including a selector uniformly responsive only within a narrow frequency band, tuning means for varying the frequency difference between a desired signal carrier and the middle of said band, auxiliary means for determining the signal output of the receiver, and means including a selective network more sharply tuned than said selector and controlling said auxiliary means as a a function of said frequency difference for causing the latter to produce maximum signal output when said frequency difference is half the width of said band.

42. In a modulated-carrier frequency signal receiver including a selector uniformly responsive only within a narrow frequency band, tuning means for adjusting the carrier frequency, and auxiliary means for controlling the output of the receiver as a function of the frequency difference between a desired signal carrier and an edge of said band for producing maximum signal output only when said frequency difference is zero.

43. In a signaling system, an amplifier producing an amplified signal output, auxiliary means responsive to said amplified signal output for producing a unidirectional voltage, means for varying the amplification in said amplifier inversely in accordance with said unidirectional voltage and means for simultaneously varying the responsiveness of said auxiliary means directly in accordance with said unidirectional voltage whereby the ratio of said unidirectional voltage to said amplified signal output is increased when the signal input to said amplifier increases.

44. In a modulated-carrier-frequency signaling system, a carrier-frequency amplifier producing an amplified carrier output, means responsiv'to said amplified carrier output for producing a unidirectional voltage, said means including an auxiliary amplifier and a rectifier, means for varying the amplification in said carrier amplifier inversely in accordance with said unidirectional voltage, and means for simultaneously varying the VVamplification in said auxiliary amplifier directly in accordance with said unidirectional voltage, whereby a change in the intensity ofthe input voltage to said carrier amplifier is caused to produce a greater rate of change in the unidirectional voltage than in said carrier output.

HAROLD A. WHEELER. 

